1. Field of the Invention
This invention relates to lubricating oil compositions suitable for use in internal combustion engines.
2. Background
Contemporary lubricants such as engine oils use mixtures of additives such as dispersants, detergents, inhibitors, viscosity index improvers and the like to provide engine cleanliness and durability under a wide range of performance conditions of temperature, pressure, and lubricant service life.
Lubricating oil compositions use a variety of detergents to minimize varnish, ring zone deposits, and rust by sobulizing oil insoluble particles. Overbased detergents are used to help neutralize acids that accumulate in lubricating oil during use.
A typical detergent is an anionic material that contains a long chain oleophillic portion of the molecule and a smaller anionic or oleophobic portion of the molecule. The anionic portion of the detergent is typically derived from an organic acid such as a sulfur acid, carboxylic acid, phosphorous acid, phenol, or mixtures thereof. The counter ion is typically an alkaline earth or alkali metal. Salts that contain a substantially stochiometric amount of the metal are described as neutral salts and have a total base number (TBN; measured by ASTM D2896, TBN is defined as mg KOH/g) of from about 0 to 80. Many compositions are overbased, containing large amounts of a metal base that is achieved by reacting an excess of a metal compound (a metal hydroxide or oxide, for example) with an acidic gas (such as carbon dioxide). The resulting overbased detergent is an overbased detergent that will typically have a TBN of 150 or higher, often 250 to 450 or more.
Typical detergents include the alkali or alkaline earth metal salts of sulfates, phenates, carboxylates, phosphates, and salicylates.
U.S. Pat. No. 5,458,790 discloses preparation and use of alkaline earth metal hydrocarbyl salicylate detergents with a TBN of 300 or more. Japanese Patent Application 10053784-A describes a lubricating oil composition for diesel engines which contains base oil, 0.04-0.2 weight percent calcium as calcium salicylate with a basicity of 100 mg KOH/g or higher, 0.01-0.1 weight percent calcium salicylate or calcium phenate with a basicity of less than 100 mg KOH/g, and at least 0.02 weight percent nitrogen as polyalkenyl succinimide.
With engines increasingly demanding higher performance, there is a need for detergents that provide increased friction reduction, detergent film maintenance, and engine cleanliness.